FIG. 12 shows a mold clamping apparatus used for a prior art injection molding machine. In the figure, a mold clamping apparatus 101 comprises a base frame 102, fixed die plate (platen) 105 fixedly mounted on the base frame 102 and fitted with a fixed mold 103, movable die plate (platen) 111 movably provided in the right and left directions in the figure on a rail 107 by action of a hydraulic cylinder 113 and plurality of tie bars 115 that can link together the fixed die plate 105 and movable die plate 111. The fixed die plate 105 has therein a hydraulic cylinder 117 for mold clamping concentrically provided with each of the tie bars 115. Each of the tie bars 115 has its one end fitted with a ram 119 of the hydraulic cylinder 117.
Function of the mold clamping apparatus 101 will be described. First, the movable die plate 111 is moved to a position shown by two dotted chain lines in the figure by action of the hydraulic cylinder 113 for mold opening and closing to thereby cause the movable mold 109 to abut on the fixed mold 103. Next, an external thread portion (link portion) 121 of each of the tie bars 115 and a half nut 123 fitted to the movable die plate 111 are caused to engage with each other to thereby fix the movable die plate 111 to the tie bars 115. Then, pressure of working fluid in the hydraulic cylinder 117 is elevated so that the fixed mold 103 and movable mold 109 are pressed against each other at a mold mating face. Thus, a mold clamping is completed and a molten resin is injected into a cavity in the mold from an injection unit 125 so that a molded article is produced.
Here, there are shortcomings, as follows, in the above-mentioned mold clamping apparatus. That is, as shown in FIG. 13, when the fixed mold 103 and movable mold 109 are clamped together by the hydraulic cylinder 117 supplied with the high pressure working fluid, a large tensile load T acts on each of the tie bars 115 and thereby the fixed die plate 105 and movable die plate 111 are bent by force F as shown by broken lines there. This often generates a small gap S at the contact face between the fixed mold 103 and movable mold 109 and there is a possibility that the gap S causes a resin leakage or forms molding flashes on the molded articles. Especially, in case of a large size mold clamping apparatus, as the cavity volume in the mold is large, this tendency becomes evident.
This problem may be solved by increasing a die plate thickness so as to suppress the above-mentioned bending of the die plate but, on the other hand, weight of the die plate becomes large and this invites a manufacturing problem and an increase of manufacturing cost. Moreover, the heavier weight causes a transportational problem.
In order to solve these problems, in a mold clamping apparatus of the below-mentioned Patent Document 1, a supporting mechanism comprises a first element to fix separately arranged end portions of tie bars to each other and a second element to connect a central portion of a stationary platen to the end portions of the tie bars with a certain angle relative to a plane of the stationary platen and the stationary platen is fitted to the tie bars via the supporting mechanism, wherein a protruding end of the second element of the supporting mechanism supports the central portion of the stationary platen or supports the stationary platen within a projected plane of a fitting face of the mold. Thereby, even if a mold having a small fitting area is used, generation of flexure of the stationary platen is prevented. However, the supporting mechanism of this Patent Document 1 is complicated and there is a possibility that the manufacturing cost becomes high.
In a mold clamping apparatus of the below-mentioned Patent Document 2, a clamping force conveying portion conveying a clamping force from a hydraulic cylinder to a mold fixing member is so formed that an outer periphery of a conveying face of the clamping force conveying portion is positioned on the inner side of an outer periphery of a force receiving face of the mold fixing member. By so forming the clamping force conveying portion relative to the mold fixing member, the clamping force is conveyed not to the outer peripheral portion of the mold fixing member but to a central axis side of the mold fixing member. Thereby, strain of the mold fixing member, especially strain of the central side of the mold fixing member, can be reduced and the movable mold and fixed mold can be securely clamped together so that no small gap is generated. Nevertheless, in the Patent Document 2, no concrete construction of the die plate (platen) is disclosed.
Also, in a mold clamping apparatus of the below-mentioned Patent Document 3, such a light weight type fixed platen is disclosed as to have a fixed mold supporting body portion comprising a boss portion, surrounding frame portion, radial ribs connecting together the boss portion and frame portion and mold fitting wall. This fixed platen could be considered to be used as a fixed die plate for the mold clamping apparatus of the Patent Document 2. However, in case of the fixed platen as so constructed, if it is clamped by the tie bars, the fixed platen will be bent and the force acting on the connecting ribs at that time becomes stress acting in the tension direction.
Usually, an allowable stress of materials corresponding to the stress in the tension direction is small as compared with an allowable stress acting in the compression direction. For this reason, thickness of the connecting ribs cannot be made smaller and an optimal weight alleviation cannot be realized.
Moreover, in a mold clamping apparatus of the below-mentioned Patent Document 4, a mold fitting wall is supported by an arch-shaped or C or V-shaped intermediate supporting structure or a conical or spherical intermediate supporting structure. When a fixed mold and movable mold are clamped together, force generated on a mold mounting face is dispersed outward (toward a tie bar side) by the intermediate supporting structure so that the mold mounting face may not be ununiformly flexed. However, the die plate (platen) structure used for the mold clamping apparatus of the Patent Document 4 has a special shape. Hence, manufacture thereof is difficult and there is a problem to invite a high manufacturing cost.
In addition to the above-mentioned problems, recently, there is a tendency that molded articles produced by injection molding become large in size and structures thereof become complicated and in case of molding an automobile bumper or the like, for example, there is seen a new kind of problem of “mouth opening” generated by a lateral directional deformation of the mold side wall.
FIG. 14 is an explanatory view of this problem. In FIG. 11, numeral 150 designates a schematically illustrated automobile bumper, wherein a front face portion 150a has its both ends bent nearly with a right angle and fitted with side face portions 150b extending comparatively long, so that when seen from above, a plan view of the bumper is a C-shaped form. Numeral 151 designates a female mold as the fixed mold (see 103 in FIG. 13) fitted to the fixed die plate (see 105 in FIG. 13), wherein a front portion 151a of rectangular shape and side wall portions 151b bending nearly With a right angle on both ends of the front portion 151a are formed. Numeral 152 (shown by two dotted chain lines) designates a male mold (movable mold) forming a pair of molds together with the female mold 151. A seat plate for fitting the male mold 152 on the lower left hand side is omitted in the illustration.
In case where a molded article is to be produced by injection molding using such a mold, when a molten resin is injected into a cavity in the mold (that is, a space occupied by the bumper 150 in the figure), the mold receives an outward pressure from the resin. Thereby, the side wall portion 151b of the female mold 151 has its both terminal end sides deformed (on the lower left hand side in the figure), as exaggeratedly shown by two dotted chain lines, to open outward in the right and left directions, so that a gap of triangular shape (mouth opening) is generated at the contact face with the male mold 152. For this reason, thickness of the side face portion 150b of the molded bumper 150 is increased and flashes are generated by the resin leaking into a gap 153 formed in upper and lower portions of the side face portion 150b. 
As this deformation quantity is small, while the thickness increase of the bumper 150 is not very problematic, there is a problem in that much work is required for removal of the flashes. Usually, while the mold is manufactured with a sufficient strength, as the side wall portion 151b of the female mold 151 long projects from the front portion 151a in a cantilever form, the bending deformation becomes large. Hence, by increasing the thickness so as to further enhance the rigidity, the deformation can be prevented. Nevertheless, if a rigidity design is to be sought, the mold becomes large and there will arise other problems.                (Patent Document 1) Japanese laid-open patent application 1996-258103        (Patent Document 2) Japanese laid-open patent application 2001-1381        (Patent Document 3) Japanese laid-open patent application 2003-25378        (Patent Document 4) Japanese patent 2858647        